Microwave applicator and method for the surface scarification of contaminated concrete

ABSTRACT

The applicator directs microwaves through a guide (5) and housing to focus microwaves generated from a microwave emitter to the surface of a target of contaminated concrete. The housing (4) of the applicator has a elliptical base and a truncated elliptical section with two focal areas (F1) and (F2). A component (11) is arranged at the focal area (F1) in the direction of the waveguide to diffuse incident waves to a wall of the housing where they are reflected to the focusing area (F2).

DESCRIPTION

The invention pertains to a micro-wave applicator and method for thesurface scarification of contaminated concrete.

The dismantling of used nuclear installations involves the destructionof the contaminated equipment and in particular its breaking up intopieces which are then put into drums of bitumen that are stored inspecialized installations. Concrete walls that must be subjected to thisbreaking up treatment pose a particular problem however because of theirthickness: since the contamination is absorbed into the surface layersof the concrete, the heart of the concrete remains clean and no specialtreatment can be justified for it.

The interest in not excessively increasing the volume of material to bestored has lead the industry to separate the contaminated layer from theremainder of the concrete walls by techniques called "crust removal"techniques and which consist of carrying out surface scarification ofthe concrete walls which leaves the clean heart of the concrete in placebut detaches the contaminated layer. A number of purely mechanical toolshave been used to do this such as bush hammers, pneumatic picks andpressurized water jets as well as the application of micro-waves. Thisnew technique makes use of the presence of water in the concrete, whichis heated by the micro-waves so that it boils and explodes within thesolid material. This produces the desired scarification.

The correct implementation of the method rests however on suitablychoosing certain parameters such as the power of the micro-waves, theirfrequency their area of application and their direction. In an apparatusdescribed in the article "Microwave system for the removal of concretesurface layers" by P. Corleto and co-workers and published by theItalian Agency for New Technologies for Energy and the Environment(ENEA), several high power magnetrons were used to emit micro-wavesspread out over a large surface area. Hence a large volume of concretewas heated all the more since the micro-waves, whose frequency wasrelatively low at 2450 MHz in the equipment proposed, penetrated moredeeply into the concrete. This apparatus appears to be effective, butone may suppose that other ways of proceeding would also be suitable,ways which allow one to remove the crust from the same area of concretewhile using much less power so as to prevent the divergence of themicrowaves which leads to a reduction in the power per unit volumedeposited in the concrete and eventually to decreased efficiency of themethod. This invention has been designed to take account of theseconsiderations and its essential characteristic consists in that themicrowaves are focused onto a more or less limited or more or lessslender area, within which the heating is concentrated, and whichdetermines the depth of the crust removed once the focusing area hasbeen stabilized.

The known equipment for removing the concrete crust have an appliancethat includes a head through which the microwaves leave a waveguide.This application head, placed on the concrete wall or positioned at asmall distance from it, must therefore be designed to create the desiredfocus. Although the focusing of waves does not appear to have beenproposed in the technique we are concerned with, it is known thatappliances have been developed for medical purposes to focus microwavesand to create localized hyperthermia in the body of a patient, forexample to destroy a tumor at the focal point. Three different pieces ofequipment are described in the articles "A Direct Contact Microwave LensApplicator with a Microcomputer Controlled Heating System for LocalHyperthermia" by Nikawa and others (IEEE Transactions on MicrowaveTheory and Techniques, Vol. MTT-34, No. 5, May 1986), "An Electric FieldConverging Applicator with Heating Pattern Controller for MicrowaveHyperthermia" again by Nikawa and others (same source) and "MicrowaveApplicator using Two Slots on Sphere" by Krairiksh and others, publishedby IEEE and presented at the Asia-Pacific Microwave Conference atAdelaide in 1992. One of these pieces of equipment includes a convergentlens at the waveguide outlet. Another includes a waveguide providing afinal broadening and which is divided at that place by parallel plateswhich are exposed to a beam of microwaves made divergent on entering thebroadening section. Wave reflections produced at the parallel platesmake the beam convergent and focused at the applicator outlet. Finally athird carries a hemispherical application head from which the microwavesleave through the slots in an arc of a circle. Satisfactory focusing isprovided by these systems, but for removing the crust from concreteanother kind is desirable since these application heads with an openingof large surface area can be easily damaged by the dust and debrisdetached from the scarified concrete. It should be noted that the largeopenings also imply a loss of efficiency since the proportion ofmicrowaves reflected by the concrete to the outside and which aretherefore lost, is greater.

The essential objective of the invention is therefore an applicatorhaving a head made up of a housing fitted with an outlet opening for themicrowaves, which allows a beam of microwaves to be focused on a targetwith good sharpness, without the structure being complicated and withoutthe opening being large.

Since the main application envisaged of removing a concrete crustrequires wide ranging treatment of walls of extended area which willinvolve large expenditures of energy, it is desirable to restrictmicrowave leakage at the applicator outlet and above all microwavereflections returning into the waveguide, in the direction of themicrowave producing apparatus which could then easily be damaged.

These various problems have been resolved with an applicator comprisinga waveguide and a head in which this waveguide terminates, the headhaving an opening directed towards a target for the microwaves and whichessentially comprises a housing which reflects the microwaves. Two mainvariants are proposed which provide the common original element that thehousing is a surface with a truncated elliptical section and has twofocal areas, one of which situated outside the opening is the focalpoint for the microwaves. The other focal area is a place for dispersionof microwaves coming from the waveguide towards the surface of thehousing where the microwaves are reflected to converge towards thefocusing area. In one of the variants, the focusing area for dispersionis occupied by a component that reflects the microwaves and thewaveguide is directed towards this component, which can be spherical,cylindrical or with the shape of a dihedral at an angle directed towardsthe waveguide. In the other embodiment, there is no reflection to theinternal focal area, but diffusion of the microwaves which leave thewaveguide at this place. A diffusing component equipped with multipleslots in which the waveguide ends can be provided.

The main application envisaged is therefore the removal of acontaminated concrete crust, but it is not the only application and theinvention could find uses in the grinding of stones or in medicine.

The invention will now be described with the help of the followingfigures, which give some examples of its implementation:

FIG. 1 represents a general view of the invention

FIG. 2 represents a first design of the applicator

FIG. 3 illustrates another view of this design of the applicator

FIG. 4 illustrates a second design of the applicator

FIG. 5 illustrates a third design of the applicator

FIGS. 6 and 7 illustrates variants of the embodiments in FIGS. 2 and 4.

The apparatus can be mounted on a carriage 1 that moves over theconcrete wall 2 which is to have its crust removed; it comprises amicrowave generator 3, an application head 4, a waveguide 5 linking thetwo preceding components, an aspirator 6, an aspiration dish 7surrounding the end of the application head 4, an aspiration pipe 8 thatends in the dish 7, and if the need arises a membrane 9 that blocks theopening 10 of the application head 4. The microwaves originating fromthe emitter 3 pass through the waveguide 5 and leave the applicationhead 4 through the opening 10, placed directly on the wall 2 or at avery small distance from it so as to restrict leakage; the optionalmembrane 9 is used to protect the interior of the application head 4from dust and debris produced by the crumbling of the concrete, butobviously it is permeable to the microwaves. The dust and the debris goup into the dish 7 and are aspirated by the aspirator 6.

Referring to FIGS. 2 and 3, it can be seen that the application head 4is a housing with the shape of a truncated cylinder with an ellipticalbase or an ellipsoidal cylinder, which includes two focal areas F1 andF2 and in which the truncation is such that the second focal area F2 issituated outside the housing, beneath the surface of the concrete wall2. The waveguide 5, which can be formed by a metal sheath of rectangularcross section, has a plane of symmetry which coincides with the planethat links the focal areas F1 and F2 of the ellipsoidal cylinder. Thefirst area F1, situated within the housing, is occupied by a reflector11, in this case formed by a metal cylinder connected to the applicationhead 4. The waves M, the path of one being shown by arrows, leave thewaveguide 5 parallel to the plane of symmetry of the housing and arethen reflected by the reflector 11 to the surface of the applicationhead 4, which reflects them in its turn to the focal area F2 whatevertheir initial path and in particular their point of reflection on thereflector 11: the focusing is almost perfect, thanks to the geometricproperties of the ellipsoidal cylinder and it is only spoiled by thediameter of the reflector 11 not being zero, which prevents reflectionat the first focal area F1 itself. The second focal area F2 is, inreality positioned a little deeper in the concrete wall 2 than is shown,because of the refraction of the waves produced at the interface betweenthe air and the concrete. The air-concrete interface also causesreflections of waves in all directions, in particular towards thewaveguide 5 and towards the outside. The first of these however does nothave a damaging effect since the reflector 11 stops the greater part ofthem and therefore protects the waveguide 5; and the second of these isreduced because of the contracted aspect of the opening 10.

The interesting feature of the housing in the shape of a cylinder withan elliptical base is that it can be given a width significantly greaterin the direction that is transverse to its movement 1, so as to spreadthe heating effect over a bigger width of concrete and scarify it alongbroader bands. FIG. 3 shows that the focal areas F1 and F2 are linearand have the appearance of segments parallel to the axis of thereflector 11 in an ellipsoidal cylinder, and that the focal area F2represents the width of the band on the concrete that is heated. Therectangular waveguide 5 has a section, the larger side of which isparallel to the transverse direction in order to emit the waves over agreater width.

However as the cylindrical reflector 11 has the disadvantage of sendingback part of the radiation which is in practice coincident with thelarge axis of the ellipse towards the waveguide 5, which could damagethe emitter 3, it could be advisable to replace it with the dihedralshape 14 in FIG. 4, made up of two flat facets 15 linked by an angle 16directed towards the waveguide 5 and opening out towards the opening 10.In this way reflections normal to the waveguide 5 which are reflectionswhich are dangerous for the emitter, are prevented. The disadvantage ofthe dihedral shape 14 is however that it focuses the radiation towardsthe second focal point F2, less well.

Yet another design can be proposed, in which the waveguide 5 coaxialwith the application head 4 is replaced by a waveguide 20 coaxial withthe first focal area and situated in its extension. The waveguide 20extends into the inside of the housing and has the appearance of a tube17 pierced with fine radial slots 18 extending along its length anddistributed over a large part of its surface, except towards the opening10. The waves leave the tube 17 in all directions starting from thefirst focal point F1. As in the preceding embodiments, they arereflected from the internal surface of the application head 4 towardsthe second focal area F2. As the wave path remains the same aspreviously from the first focal area F1, the operation of the equipmentremains the same.

The ellipsoid of the application head 4 can have a transverse dimensionmore or less large: the preceding Figures have illustrated the case ofwide application heads 4, a reflector 11, 14 or 17 that is lengthenedtransversely and with linear focal areas F1 and F2; application headswith rotational symmetry can also be chosen, the focal areas being thenreplaced by restricted focal points; the reflective component will be asphere or a cone which will replace the cylinder 11 or the dihedralshape 14, and it will be linked to the application head 4 by suspensionarms; the drawings in FIGS. 2 and 4 then remain valid, all the sectionsof the application head then having a truncated elliptical section

FIGS. 6 and 7 finally illustrate variants to the embodiments in FIGS. 2and 4, which are distinguished in that the waveguide 5, instead ofemerging at the internal surface of the housing of the application head4, comprises an extension 19 which disappears into the chambersurrounded by the application head 4 towards the focal area F1; theextension ends at a distance that is advantageously close to a quarterof the wavelength of the microwaves. It has been observed that thisarrangement gives good focusing results thanks to the longer guidancefor the microwaves. It has also been observed that it was advantageousthat the reflector 11 or 14 has a small size roughly equal to a quarterof the wavelength. These values are however approximate and result fromempirical tests, so that other good solutions, indeed better solutionsmay exist in particular cases notably in relation to the shape of thereflector.

What is claimed is:
 1. A microwave applicator for focusing microwavesgenerated from a microwave emitter, said applicator comprising awaveguide (5) and a head (4) in which the waveguide terminates, the headhaving an opening (10) directed towards a target (2) for the microwavesand essentially comprising a housing that reflects the microwaves,characterized in that the housing is of a truncated elliptical sectionhaving two focal areas, one of the focal areas (F1), towards which thewaveguide is directed, being occupied by a component (11) that reflectsthe microwaves and the other of the focal areas (F2) being situatedoutside the opening.
 2. A microwave applicator according to claim 1,characterized in that the component that reflects the microwaves isspherical.
 3. A microwave applicator according to claim 1, characterizedin that the component that reflects the microwaves is cylindrical (11).4. A microwave applicator according to claim 1, characterized in thatthe component that reflects the microwaves has a dihedral shape (14)with an angle directed towards the waveguide.
 5. A microwave applicatoraccording to claim 1, characterized in that the component that reflectsthe microwaves is a cone.
 6. A microwave applicator according to claim1, characterized in that the waveguide (5) emerges at an internalsurface of the housing that reflects the microwaves.
 7. A microwaveapplicator according to claim 1, characterized in that the waveguide (5)comprises an extension (19) into the housing that reflects themicrowaves and which ends at a distance close to a quarter of thewavelength of the microwaves from the focal area (F1) occupied by thecomponent that reflects the microwaves.
 8. A microwave applicator forfocusing microwaves generated from a microwave emitter, said applicatorcomprising a waveguide (20) and a head (4) in which the waveguideterminates, the head having an opening (10) directed towards a target(2) for the microwaves and essentially comprising a housing thatreflects the microwaves, characterized in that the housing is of atruncated elliptical section having two focal areas, one of the focalareas (F1), at which the waveguide terminates, being occupied by acomponent (17) that diffuses the microwaves and the other of the focalareas (F2) being situated outside the opening (10).
 9. A microwaveapplicator according to claim 8, characterized in that the componentthat diffuses the microwaves is a component cut with slots (18) and inwhich the waveguide (15) ends.
 10. A method for focusing microwavesgenerated from a microwave emitter at the surface of a target ofcontaminated concrete comprising the steps of:directing the microwavesthrough a waveguide towards the surface of said target; terminating thewaveguide in a housing having an opening facing the target and atruncated elliptical section with two focal areas (F1) and (F2) with thefocal area (F1) disposed in the direction of the microwaves emitted fromsaid waveguide and with the focal area (F2) forming a focal point forthe microwaves outside the opening adapted to lie beneath the surface ofthe target of contaminated concrete and; interposing a member in thehousing at the focal area (F1) to cause incident microwaves to bereflected or diffused in the direction of the housing so that themicrowaves converge at the focal area (F2) to cause surfacescarification of the contaminated concrete.